Floating gate memory device having discontinuous gate oxide thickness over the channel region

ABSTRACT

A method and structure for a programmable read-only memory comprises a thin gate oxide over a source region and a thick gate oxide over the drain region. A semiconductor substrate is lightly doped and has regions of thin sacrificial oxide overlying what will become the transistor channel, source, and drain, and thick field oxide. The region that will become the transistor source is protected with photoresist, and the drain region and a portion of the channel is doped, for example, with boron. The resist and sacrificial oxide is stripped, and gate oxide is formed from the exposed silicon substrate. The more heavily doped drain and channel regions oxidize at a faster rate than the lightly doped source region, and thus the gate oxide formed is thicker. Floating and control gates are formed over the channel region, covering both the thicker and thinner gate oxide. The cell resulting from the process has increased coupling coefficient, easier programmability, and better storage of the charge on the floating gate than a conventional cell.

This application is a continuation of application Ser. No. 08/104,753filed Aug. 11, 1993, now abandoned.

FIELD OF THE INVENTION

The invention relates to the field of semiconductor manufacture, andmore specifically to the manufacture of nonvolatile memories.

BACKGROUND OF THE INVENTION

Erasable Programmable Read-only Memories (EPROMs), Electrically ErasableProgrammable Read-only Memories (E² PROMs), and Flash E² PROMs(hereafter collectively, PROMs or nonvolatile memory devices) haveseveral structures which allow them to hold a charge without refresh forextended periods of time. FIG. 1 shows a top view of a PROM array, FIG.2 shows a cross section along "AA" of FIG. 1, and FIG. 3 shows a crosssection along "BB" of FIG. 1. The charge itself is stored on a "floatinggate" 10 also referred to as Poly 1 or P1, which is a structure ofpolycrystalline silicon (hereafter, poly) surrounded on all sides by alayer of oxide 12. Located superjacent and parallel to this P1 structureis another poly structure, the "control gate" 14 or P2. P1 10 and P2 14function as two plates of a capacitor. Below the P1 layer are two N+junctions, one which functions as the transistor source 16 and the otheras the drain 18, which are doped into a P-type substrate 20. The portionof the substrate 20 between the source 16 and the drain 18 is thechannel 22. The cell functions as an enhancement-type N-channel metaloxide semiconductor field effect transistor (MOSFET) with two gates ofpoly.

There are structures that make up a PROM array which are common toseveral transistors in the array. FIG. 1 shows the transistor sources16, drains 18, digit lines 24, floating gates 10, and control or "word"lines 26 which form control gates 14 as they pass over the floatinggates 10. Also shown as a dotted line is the "active area" 28interspersed with areas of field oxide 30. A single word line 26 iscommon to all transistors in a single column and functions as a controlgate 14 for all transistors in the column. When the word line isselected it activates all transistors in the column. The source regions16, which run parallel with the control lines 26, are common to alltransistors in two adjacent columns. Individual transistor drains 18 arecommon to two transistors in adjacent columns. The digit (or bit) lines24 are common with the drains 18 of all transistors in a single row.

The potential which must be applied on the control gate to turn on thetransistor is much higher in a device storing a charge than in a devicewhich does not have a potential stored on P1. To read the content of thefloating gate, a potential somewhere between the low and high potentialvalues (i.e. the threshold voltage V_(t)) is applied to the controlgate. If a cell is set to a zero, arbitrarily defined by storing -3 V onthe floating gate 10, and 5 V is applied to the control gate, the netdifference in potential which effects the transistor channel is notgreat enough to allow the electrons to pass from the source to thedrain. Assuming a 0.7 coupling coefficient (described below) the effecton the transistor is defined as -3.0 V +(5 V×0.7)=0.5 V. If V_(t) is 1.0V, the transistor will not trip. If a cell is set to a one, arbitrarilydefined by storing 0V on the floating gate 10, the 5 V differencebetween the control gate and the source will affect the channel greatlyenough to allow electrons to pass between the source and drain. Assumingthe same 0.7 coupling coefficient, the effect on the transistor isdefined as 0 V+(5 V×0.7)=3.5 V. After the control gate 26 is activated,each cell along that control gate 26 outputs the cell information ontheir respective digit lines 24, either an OFF if the floating gate isstoring a -3.0 V or an ON if the cell is storing 0.0 V. The informationon the digit line 24 which corresponds to the cell to be read isobtained with a sense amplifier (not shown), with one sense amp for eachdigit line.

In a conventional Flash E² PROM cell, the floating gate and the controlgate are both manufactured from N-type poly. The floating gate andcontrol gate are both highly doped with N+ material. The substrate isP-type conductivity with N+ junctions which form the source and drainregions. To manufacture N-type poly, a poly structure is doped withatoms having more than four valence electrons (group V or higher), suchas arsenic or phosphorus, which introduces negatively charged majoritycarriers into the silicon and makes the semiconductive material muchmore conductive than nondoped poly due to electron conduction.

There are many ways to program a PROM. In one technique, a potentialsuch as 12 V, for example, is applied on the control gate.Simultaneously, a voltage pulse, for example 8 V, is applied betweensource and drain. The large positive potential on the control gateestablishes an electric field in the insulating oxide. This electricfield attracts the electrons generated from the so-called "avalanchebreakdown" or "hot electrons" of the transistor due to the high drainand control gate potentials, and accelerates them toward the floatinggate, which they enter through the oxide dielectric. In this way thefloating gate is charged, and the charge that accumulates on it becomestrapped.

The amount of electron flow is determined partly by the thickness of thegate oxide. If the gate oxide is very thin, the electron flow can berelatively large. In this case, a lower voltage can be applied on thedrain and control gate. If the oxide is thicker, the electron flow issmall and a large potential must be applied to the drain and controlgate to influence the electrons to pass through the gate oxide to thefloating gate.

Another programming method is by Fowler-Nordheim tunneling. By applyinga high potential on the word line and grounding the source and/or thedrain, electrons will tunnel through the thin gate oxide to the floatinggate. Fowler-Nordheim tunneling is critically determined by thepotential between the control gate and the source and/or drain (andtherefore the electric field) and the thickness of the gate oxide.

To return the floating gate from a charged state to a state with nocharge (erase), the electrons are caused to return to the substrate. Inan EPROM, this is accomplished with ultraviolet light which excites theelectrons past a certain energy state, thereby allowing them to passthrough the oxide and return to the substrate. In an E² PROM, thisexcitation is accomplished with an electrical field.

The floating gate of an E² PROM forms a capacitor with the control gate,as described above, and also with the source, the drain, and thechannel. This arrangement is described in FIG. 4. The couplingcapacitances associated with P1 are described by the couplingcoefficient, which is depicted by the equation (referring to FIG. 4):##EQU1## where CC is the coupling coefficient, C1 represents thecoupling between P1 and P2, C2 represents the coupling between P1 andthe source, C3 represents the coupling between P1 and the drain, and C4represents the coupling between P1 and the channel. As can be determinedfrom the equation, the coupling coefficient can never reach the idealstate (1.00) since the capacitance between the floating gate and thecontrol gate is always divided by itself plus some additionalcapacitance. Still, the goal of designers is to bring the couplingcoefficient as close to unity as possible.

The coupling between the floating gate and the channel (C4 in theequation above) is partially determined by the thickness of the gateoxide under the floating gate. Thinning the oxide has variousadvantageous and detrimental effects on the functionality of the cell.With a relatively thin gate oxide, which can range from about 100 Å toabout 150 Å with current technology, the cell requires less voltage toprogram. The thinner gate oxide allows an easier passage of theelectrons from the source (or drain) region to the floating gate,thereby requiring less voltage to program the array. In addition torequiring less power, the programming of one cell does not as greatlyaffect an adjacent cell (less disturbance), and the data is morereliably written into the cell with the thinner gate oxide. Thinning thegate oxide, however, increases the capacitance between the floating gateand the substrate as the two "plates" of the capacitor are closertogether. As seen in the equation above, this decreases the couplingcoefficient. A thin gate oxide, while an aid in programming, adverselyaffects the storage of the charge on the floating gate for the samereason, that electrons can more easily pass through the gate oxide, andcan therefore discharge from the floating gate to the substrate.

Conversely, a cell with a thicker gate oxide, for instance from about150 Å to about 250 Å, has a larger distance between the two plates ofthe capacitor (the floating gate and the transistor channel region) anda coupling coefficient which is not adversely affected by C4. This cellhas a thicker tunneling dielectric, and therefore an excellent storageof the charge on the floating gate as the charge does not easily leakfrom the floating gate to the substrate through the thick gate oxide.However, it is difficult to program this cell, and programming requiresa higher potential. This increases power consumption and the programmingof one cell can adversely affect an adjacent cell.

Attempts have been made to create a cell with a large percentage ofthick gate oxide under the floating gate for good storage of the charge,and an area under the floating gate having thin gate oxide to allow forlow voltage programming. While a cell with these specificationsfunctions effectively, attaining this structure reliably has proven tobe difficult, especially with high density devices. A more easilymanufacturable PROM cell having the advantages of both thick and thingate oxide would be a desirable structure.

SUMMARY OF THE INVENTION

An embodiment of the inventive method comprises regions of field oxideand sacrificial oxide over a lightly doped semiconductor substrate. Apatterned layer of photoresist is formed over a portion of thesacrificial oxide, while a second portion of the sacrificial oxide isleft exposed. The wafer surface is doped, and the lightly dopedsemiconductor substrate is further doped through the exposed sacrificialoxide. This forms a relatively heavier doped area in the semiconductorsubstrate, while the substrate remains lightly doped under the resistlayer. This implant can be performed at the same time as the channelstop implant.

Next, the photoresist is removed as is the sacrificial oxide layer. Thisexposes the semiconductor substrate. The lightly doped substrate and themore heavily doped substrate are oxidized. The more heavily dopedregions of the substrate oxidize at a faster rate, and thus result in athicker gate oxide in those regions. The lightly doped substrateoxidizes at a slower rate than the more heavily doped substrate, and theoxidized regions resulting therefrom are therefore thinner.

Finally, floating and control gates are formed over the thick and thingate oxide. The portions of the floating and control gates that overliethe thick gate oxide are further away from the substrate than theportions of the floating and control gates that overlie the thin gateoxide.

Other advantages will become apparent to those skilled in the art fromthe following detailed description read in conjunction with the appendedclaims and the drawings attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a PROM array;

FIG. 2 is a cross section of the FIG. 1 PROM array along "AA";

FIG. 3 is a cross section of the FIG. 1 PROM array along "BB";

FIG. 4 shows the capacitances associated with the cell structures of aPROM;

FIG. 5 is a cross section showing a first step of one embodiment of theinvention showing the doping of the underlying substrate;

FIG. 6 is a cross section showing an intermediate step of one embodimentof the invention;

FIG. 7 shows the FIG. 6 structure after the formation of the gate oxide;and

FIG. 8 shows the FIG. 7 structure after the formation of the floatingand control gates.

It should be emphasized that the drawings herein are not to scale butare merely schematic representations and are not intended to portray thespecific parameters or the structural details of the invention, whichcan be determined by one of skill in the art by examination of theinformation herein.

DETAIL DESCRIPTION OF THE INVENTION

In accordance with one embodiment of the invention as shown in FIG. 5, alightly doped semiconductor substrate 50 is formed having layers offield oxide 52 and sacrificial oxide 54. The semiconductor substrate 50is doped with boron or other p-type dopant in the active area to animpurity concentration of about 1E16 ions/cm⁻³ to about 5E17 ions/cm⁻³during the course of wafer fabrication before FIG. 5. The field oxide 52generally has a thickness of between about 3000 Å and about 6000 Å, andthe sacrificial oxide 54 has a thickness of between about 150 Å andabout 500 Å, although other thicknesses would function sufficiently.

Next, a mask 56, for example of photoresist, is formed over what willeventually function as the transistor source region and a portion ofwhat will be the channel region. What will form the transistor drainregion is left exposed as shown in FIG. 5. FIG. 5 shows about 50% of thechannel region covered by the resist but other percentages would also bepossible and sufficient to alter the electrical characteristics of thetransistor. For example, from about 10% to 90% of the channel regioncould be covered with thick oxide to provide a cell having alteredelectrical characteristics and the advantages of the increased gateoxide thickness.

During the formation of a conventional structure, a field implant maskis formed over the entire active area, and the substrate is implantedthrough the field oxide. This mask could be shortened somewhat to leavethe transistor drain region and a portion of the channel region exposed,and therefore no additional mask steps would be required for theinventive cell structure.

In any case, the wafer surface is doped, for example with boron, asshown in FIG. 5. The exposed channel and drain regions 58 are therebymore heavily doped with boron than is the covered source and channelregions 60. For example, the exposed active area 60 is doped with boronto an impurity concentration of about 1E18 ions/cm⁻³ while the coveredsource and channel regions 60 were originally doped to a concentrationof between about 1E16 and about 5E17 ions/cm⁻³. After further doping theexposed regions of the active area 58 the mask 56 and sacrificial oxide54 are removed to result in the structure of FIG. 6.

The exposed semiconductor substrate is oxidized by any means known inthe art, for example thermal oxidation, to form gate oxide 70, 72 asshown in FIG. 7. The more heavily doped channel and drain regions 58form a thicker gate oxide 72 than do the more lightly doped channel andsource regions 60, as the more heavily doped regions 58 oxidize at afaster rate. For example, the gate oxide 70 over the more lightly dopedchannel and source regions 60 can be between about 100 Å and about 120Å, and the thicker oxide 72 over the more heavily doped channel anddrain regions 58 can have a thickness of between about 200 Å and about350 Å, although other thicknesses would function sufficiently. After theformation of the gate oxide 70, 72, floating 80 and control gates 82 areformed as shown in FIG. 8. Another implant may be necessary to form thesource 84 and drain 86 of the transistor, for example with arsenic toform N+ areas, as can be determined by an artisan of skill in the art.It can be seen in the FIG. 8 cross-section perpendicular with thecontrol line (26 in FIG. 1) across the field oxide 52, the channel, andthe source 84 and drain 86 regions that the control gate 82 and thefloating gate 80 overlie the thick 72 and thin 70 gate oxide.

The portion of the floating 80 and control 82 gates which overlie thethin gate oxide 70 are closer to the silicon substrate 50 than theportions which overlie the thick gate oxide 72. This results in adecreased coupling between the floating gate P1 and the transistorchannel and drain (referring to FIG. 4). Therefore, in the equation##EQU2## the terms C3 and C4 as described in the background sectiondecrease, and the overall coupling coefficient "CC" increases nearer tothe ideal value of "1". In addition, the drain region has an increasednumber of "hot electrons" due to the higher boron concentration, andtherefore the PROM is easier to program. Finally, the thicker gate oxideover the channel and drain regions provides better insulation for thefloating gate and reduces the number of electrons which will bleed offthe floating gate through the gate oxide to the substrate. Thisincreases the amount of time the floating gate can store a charge andimproves reliability of the PROM.

While this invention has been described with reference to illustrativeembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the illustrative embodiments, as well asadditional embodiments of the invention, will be apparent to personsskilled in the art upon reference to this description. It is thereforecontemplated that the appended claims will cover any such modificationsor embodiments as fall within the true scope of the invention.

What is claimed is:
 1. A programmable read-only memory cell,comprising:a) a semiconductor substrate; b) a source region formed insaid substrate; c) a drain region formed in said substrate; d) atransistor channel formed in said substrate between said source anddrain regions, said channel having a first region with a first impurityconcentration and a second region with a second impurity concentration,said second impurity concentration being higher than said first impurityconcentration; e) first gate oxide having a thickness of from about 100Å to about 120 Å formed over said first channel region; f) second gateoxide having a thickness of from about 200 Å to about 350 Å formed oversaid second channel region; g) a control gate formed over said channelregion,wherein in a cross-section perpendicular with a control lineacross said channel, said source region, and said drain region, saidcontrol gate overlies said first and second gate oxide.
 2. The memorycell of claim 1 wherein said first impurity concentration is in therange from about 1E16 ions/cm⁻³ to about 5E17 ions/cm⁻³, and said secondimpurity concentration is about 1E18 ions/cm⁻³.
 3. The memory cell ofclaim 1 wherein said second gate oxide covers at least 10% of saidtransistor channel.
 4. The memory cell of claim 1 wherein said secondgate oxide covers about 50% of said transistor channel.
 5. The memorycell of claim 1 wherein said second gate oxide covers less than about90% of said transistor channel.
 6. The memory cell of claim 1 wherein atleast 10% of said transistor channel comprises said second impurityconcentration.
 7. The memory cell of claim 1 wherein about 50% of saidtransistor channel comprises said second impurity concentration.
 8. Anonvolatile programmable read-only memory cell, comprising:a) asemiconductor substrate; b) a source region formed in said substrate; c)a drain region formed in said substrate; d) a transistor channel formedin said substrate between said source and drain regions, said channelhaving a first region with a first impurity concentration and a secondregion with a second impurity concentration, said second impurityconcentration being higher than said first impurity concentration; e)first gate oxide having a thickness from about 100 Å to about 120 Åformed over said first channel region; f)second gate oxide having athickness from about 200 Å to about 350 Å formed over said secondchannel region; g) a floating gate having first and second portions,said first portion of said floating gate being formed over said firstgate oxide and said second portion of said floating gate being formedover said second gate oxide, said first portion of said floating gatebeing nearer said substrate than said second portion of said floatinggate; and h) a control gate formed over said floating gate;wherein in across-section perpendicular with a control line across said channel,said source region, and said drain region, said floating gate overliessaid first and second gate oxide.
 9. The memory cell of claim 8 whereinsaid first impurity concentration is from about 1E16 ions/cm⁻³ to about5E17 ions/cm⁻³, and said second impurity concentration is about 1E18ions/cm⁻³.
 10. The memory cell of claim 8 wherein said second gate oxidecovers at least 10% of said transistor channel.
 11. The memory cell ofclaim 8 wherein said second gate oxide covers about 50% of saidtransistor channel.
 12. The memory cell of claim 8 wherein said secondgate oxide covers less than about 90% of said transistor channel. 13.The memory cell of claim 8 wherein at least 10% of said transistorchannel comprises said second impurity concentration.
 14. The memorycell of claim 8 wherein about 50% of said transistor channel comprisessaid second impurity concentration.
 15. A nonvolatile programmableread-only memory cell, comprising:a) a semiconductor substrate; b) asource region formed in said substrate; c) a drain region formed in saidsubstrate; d) a transistor channel formed in said substrate between saidsource and drain regions, said channel having a first region with boronions to a concentration of from about 1E16 ions/cm⁻³ to about 5E17ions/cm⁻³ and a second region doped with boron ions to a concentrationof about 1E18 ions/cm⁻³ ; e) first gate oxide formed over said firstchannel region, said first gate oxide having a thickness of from about100 Å to about 120 Å; f) second gate oxide formed over said secondchannel region, said second gate oxide having a thickness of from about200 Å to about 350 Å; g) a floating gate having first and secondportions, said first portion of said floating gate being formed oversaid first gate oxide and said second portion of said floating gatebeing formed over said second gate oxide, said first portion of saidfloating gate being nearer said substrate than said second portion ofsaid floating gate; and h) a control gate formed over said floatinggate;wherein in a cross-section perpendicular with a control line acrosssaid channel, said source region, and said drain region, said floatingand control gates overlie said first and second gate oxide.
 16. Thememory cell of claim 15 wherein said second gate oxide covers at least10% of said transistor channel.
 17. The memory cell of claim 15 whereinsaid second gate oxide covers about 50% of said transistor channel. 18.The memory cell of claim 15 wherein said second gate oxide covers lessthan about 90% of said transistor channel.
 19. The memory cell of claim15 wherein at least 10% of said transistor channel comprises said secondchannel region.
 20. The memory cell of claim 15 wherein about 50% ofsaid transistor channel comprises said second channel region.
 21. Aprogrammable read-only memory cell, comprising:a) a semiconductorsubstrate; b) a source region formed in said substrate; c) a drainregion formed in said substrate; d) a transistor channel formed in saidsubstrate between said source and drain regions, said channel having afirst region with an impurity concentration in the range from about 1E16ions/cm⁻³ to about 5E17 ions/cm⁻³ and a second region with an impurityconcentration of about 1E18 ions/cm⁻³ ; e) first gate oxide formed oversaid first channel region; f) second gate oxide formed over said secondchannel region, said second gate oxide being thicker than said firstgate oxide; g) a control gate formed over said channel region,wherein ina cross-section perpendicular with a control line across said channel,said source region, and said drain region, said control gate overliessaid first and second gate oxide.
 22. A programmable read-only memorycell, comprising:a) a semiconductor substrate; b) a source region formedin said substrate; c) a drain region formed in said substrate; d) atransistor channel formed in said substrate between said source anddrain regions, said channel having a first region with a first impurityconcentration and a second region with a second impurity concentration,said second impurity concentration being higher than said first impurityconcentration; e) first gate oxide formed over said first channelregion; f) second gate oxide formed over said second channel region andcovering about 50% of said transistor channel, said second gate oxidebeing thicker than said first gate oxide; g) a control gate formed oversaid channel region,wherein in a cross-section perpendicular with acontrol line across said channel, said source region, and said drainregion, said control gate overlies said first and second gate oxide.